In modern displays, white LEDs are used to create the white light used to backlight the LCD. It is desirable to have the ability to vary the level of the backlight used. This is desired for both maximizing contrast as well as adjusting the display to the ambient light level. Conventional LED driver circuits accomplish dimming by adjusting the on time (duty cycle) of an LED string, such that the percentage of on time creates an equivalent brightness (or average intensity) at the desired brightness.
FIG. 1 is a simplified schematic diagram of a conventional LED driver circuit for driving LED strings 102 with constant current. Operational amplifier 104 (Op1) compares the voltage across current sense resistor 108 (R1) with a reference voltage (Vref) to generate a command voltage on the gate of switching transistor 106 (T1). Vref is set at the desired voltage level for the desired LED current (Vref/R1) when the duty cycle is high. Vref is set to zero when the duty cycle is low.
Circuit 100 is commonly used because the voltage at the negative terminal of opamp 104 can be made much higher than the offset of opamp 104. Because the duty cycle controls the effective intensity for all intensity levels, the voltage at Vref is constant. Hence, we get good matching of LED string currents even at low intensity levels. A problem with circuit 100, however, is that circuit 100 places a burden on the power supply (Vsupply). Vsupply must respond to fast load changes caused by the fast edges on the duty cycle control of the LED string current. Accordingly, the conventional circuit 100 is accurate at low intensity and provides good matching on multiple LED strings, even at low intensity but creates a burden on Vsupply by requiring faster load response.
Some conventional methods modify Vref directly to set the desired brightness level for the LEDs. For example, if the value of Vref is set at 500 mV for full scale current, the value of Vref would change to 5 mV for 1% brightness. These conventional methods have the disadvantage of being susceptible to offset voltages at low currents. More important than absolute accuracy is the relative accuracy when multiple LED strings are used. Since the offsets of the current source amplifiers will not match, the currents in the LED strings will have poor matching at low current levels. Additionally, the switching transistors of the current sources must still be high voltage devices.